Define default font stack

[hedgefield]

To standardize the use of fonts in our components, we should add a default font stack. It should not contain very many custom fonts, definitely not as the first choice, so we're using the same font stack that WordPress uses. You can read more about it here, including best practices.

It renders the default system font on Mac, which is San Francisco, then falls back to Segoe UI, the default Windows font, and so on through some other fallbacks for Android, Linux etc.

font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;

Prompted by Yoast/wordpress-seo#11398 (comment) and also fixes #582